The present invention relates to a scanner, and more particularly, to an optical scanner of variable scanning speeds.
A scanner is a popular computer peripheral device. The scanner can transfer photographs, texts and pictures on a scanned object to image files. There are two different kinds of resolutions for a scanner: the horizontal resolution and the vertical resolution. The horizontal resolution, which is also called the optical resolution, represents the dot number being captured by a scanning module of the scanner on a scanned object per inch. The vertical resolution, which is also called the mechanical resolution, is determined by the relative movement speed between the scanning module and the scanned object. A conventional scanner uses a stepping motor to drive the scanning module to move relatively to the scanned object or to drive the scanned object to move relatively to the scanning module, so the vertical resolution is determined by the rotation speed of the stepping motor. As a result, the rotation speed of the stepping motor determines the vertical resolution. However, a large variation of the rotation speed may deteriorate the stability of the stepping motor and thus degrade the scanning quality. Therefore the rotation speed of the stepping motor can only be varied in a limited range and consequently the vertical resolution can only be varied within a limited range. A conventional flat bed optical scanner is taken as an example and described in the following.
Please refer to FIG. 1, which is a schematic diagram of a conventional flat bed scanner 10. The scanner 10 includes therein a stepping motor 12, an input gear 14, an output gear 16, a belt 18, a scanning module 20 and an axle 22. The scanning module 20 is connected and driven by the belt 18 to slide along the axle 22 during the scanning process. The flat bed scanner 10 uses the stepping motor 12 to drive the output gear 16 via the input gear 14. The belt 18 is driven by the output gear 16 and then forces the scanning module 20 to move relatively to the scanned object along the axle 22. To increase the vertical scanning resolution, the rotation speed of the stepping motor 12 is decreased to slow down the relative movement between the scanning module 20 and the scanned object. On the contrary, increasing the rotation speed of the stepping motor 12 will speed up the relative movement between the scanning module 20 and the scanned object, and thus decreases the vertical scanning resolution. Because a large variation of the rotation speed may deteriorate the stability of the stepping motor 12 and thus degrade the scanning quality, the rotation speed of the stepping motor 12 can only be changed in a limited range. Therefore, the vertical resolution of the conventional scanner 10 is limited within a small range.
Therefore, the present invention provides an optical scanner of variable scanning speeds. The scanner includes a scanning module, a power transmission device, a gear module and a driving device. The scanning module is connected to the power transmission device. The gear module includes at least a first gear and a second gear, and the gear module is capable of switching between a first position and a second position. The driving device drives the gear module. The driving device allows the scanning module and the scanned object to move relatively at a first speed through the first gear when the gear module switches to the first position. The driving device allows the scanning module and the scanned object to move relatively at a second speed by the second gear when the gear module switches to the second position.